DISA STIGS Viewer

AlmaLinux OS 9 SSH daemon must not allow compression or must only allow compression after successful authentication.

Overview

Finding ID Version Rule ID IA Controls Severity
V-269260 ALMA-09-020370 SV-269260r1050142_rule   Medium
Description
If compression is allowed in an SSH connection prior to authentication, vulnerabilities in the compression software could result in compromise of the system from an unauthenticated connection, potentially with root privileges.
STIG Date
CloudLinux AlmaLinux OS 9 Security Technical Implementation Guide 2025-05-22

Details

Check Text (C-73291r1048156_chk)
Verify the SSH daemon performs compression after a user successfully authenticates with the following command:

$ sshd -T | grep compression

Compression no

If the "Compression" keyword is set to "yes", this is a finding.
Fix Text (F-73192r1048157_fix)
Configure the SSH daemon to not allow compression.

Add the following line to "/etc/ssh/sshd_config", or uncomment the line and set the value to "delayed" or preferably "no":

Compression no

Alternatively, add the setting to an include file if the line "Include /etc/ssh/sshd_config.d/*.conf" is found at the top of the "/etc/ssh/sshd_config" file:

$ echo 'Compression no' > /etc/ssh/sshd_config.d/40-compression.conf

Restart the SSH daemon for the settings to take effect:

$ systemctl restart sshd.service